Neuroanatomical tracing by use of Phaseolus vulgaris-leucoagglutinin (PHA-L): electron microscopy of PHA-L-filled neuronal somata, dendrites, axons and axon terminals.

Following iontophoretic application of the plant lectin Phaseolus vulgaris-leucoagglutinin (PHA-L) to brain areas of rats, and subsequent immunohistochemistry, reaction product can be observed with the light microscope to fill neurons completely, including their somata, dendrites, dendritic appendages and axons. Moreover, axons often show profuse collateralization and indications of termination, including numerous en passant and terminal varicosities. The present report describes a protocol for combining light microscopic examination of PHA-L-stained neurons and electron microscopy of details of their processes, including the axonal varicosities. The results support the hypothesis that axonal varicosities are the light microscopic representations of synaptic axon terminals seen in electron microscopic preparations.